Double-sideband, amplitude modulated carrier signals are used in communications systems. The amplitude and/or group delay distortion of such signals after propagation through a transmission channel can, at times, render the received information unintelligible. This is especially true in radio systems wherein the transmission channel is uncontrolled and oftentimes unpredictable.
Distortion can be characterized in the frequency domain as having a linear and a nonlinear component. The linear distortion component varies directly with frequency, while the nonlinear distortion component is a more complex function of frequency which is sometimes not readily definable. Therefore, complete elimination of linear and nonlinear distortion is a difficult task. However, in many systems applications, elimination of only the linear distortion is sufficient to meet system performance objectives.
Prior art techniques to eliminate linear distortion have relied on cancellation and/or equalization techniques. In radio systems, for example, slope equalizers eliminate amplitude slope but do not equalize the group delay distortion. Furthermore, the frequency-dependent gain provided by an amplitude equalizer can result in noise enhancement. In contrast, cancellation does not rely on frequency-dependent amplification to remove the linear distortion and, therefore, does not produce any noise enhancement. However, the problem with available linear distortion cancellers is that they do not operate directly on a double-sideband, amplitude modulated carrier signal. Instead, present linear distortion cancellers operate on the baseband signals generated by demodulating the received carrier signal. When the distortion is severe, the ability to demodulate is impaired which, in turn, affects the operation of the distortion canceller. Accordingly, it would be desirable to provide a linear distortion canceller which can operate directly on a double-sideband, amplitude modulated carrier signal.